All-fiber interferometric fiber optic gyroscope having a minimum reciprocal configuration

ABSTRACT

An all-fiber interferometric fiber optic gyroscope having a minimum reciprocal configuration is described. The gyroscope comprises a polarized light source, a light detector, a light source coupler, a fiber optic loop coupler, and a polarization maintaining fiber optic loop. A first port of the light source coupler is counter-axially coupled to an output end of the polarized light source, and a second port of the light source coupler on the same side as the first port is coupled to the light detector. A third port on the other side of the light source coupler is counter-axially coupled to the fiber optic loop coupler, and the fiber optic loop coupler is counter-axially coupled to the polarization maintaining fiber optic loop. The light source splits the input polarized light and polarizes the optical signal propagated along a transmission arm alone, where the first and third ports are on the same transmission arm.

This is a continuation-in-part of International Patent Application No.PCT/CN2010/001175, filed on Aug. 3, 2010 under the Patent CooperationTreaty (PCT), which claims priority to Chinese Patent Application No.201010232656.8, filed on Jul. 16, 2010.

FIELD OF THE INVENTION

The present invention relates to all-fiber fiber optic gyroscopes, andin particular, it relates to an all-fiber interferometric fiber opticgyroscope having a minimum reciprocal configuration, which pertains tothe technical field of fiber optic sensing.

DESCRIPTION OF THE RELATED ART

A gyroscope is an instrument for measuring angular velocity-angulardisplacement in inertial space, and its history can be traced back tothousands of years ago. Stone gyroscopes were unearthed from theNeolithic relics in Xia County, Shanxi province in China. Mechanicalgyroscope is the first generation gyroscope, and laser gyroscope andfiber optic gyroscope are respectively second and third generationgyroscopes. Fiber optic gyroscopes emerged in 1976 and mass productionthereof was launched in the 1990s. The all-fiber interferometric fiberoptic gyroscope is an important variety in the family of fiber opticgyroscope, and it is widely applicable in the fields of navigation,guidance, positioning, automatic north seeking, damping on vibration fortrains or ships, orientation of antenna aiming systems, measurement ofoil well deviation, inspection of distortion and vibration oflarge-scale construction, automatic control, etc.

The optical structure of a typical all-fiber interferometric fiber opticgyroscope is shown in FIG. 1, wherein its optical elements include: asuperluminescent diode, a photodetector diode, a light source coupler, afiber optic loop coupler, a polarizer (or depolarizer), a fiber opticloop (i.e., fiber optic coil), altogether six optical elements. Theconfiguration of the six optical elements is called a minimum reciprocalconfiguration of an all-fiber interferometric fiber optic gyroscope. Thepaths between ports A-C and B-D of the light source coupler 3 aretransmission arms and have a beam splitting function. The paths betweenports A-D and B-C are coupling arms which have a light splittingfunction.

According to FIG. 1, the operation principle of the typical all-fiberinterferometric fiber optic gyroscope is: polarized light (ellipticallyor circularly polarized light) emitted from the superluminescent diode 1is input into port A of the light source coupler 3 and is split into twobeams of polarized light; wherein the beam of polarized light whichtravels along a transmission arm and is output from port C is input intothe polarizer 4; the polarizer 4 converts the input polarized light intolinearly polarized light and inputs the linearly polarized light intoport A of the fiber optic loop coupler 5. The fiber optic loop coupler 5splits the input linearly polarized light into two beams and outputsthem through ports C and D thereof; the linearly polarized light beamsoutput from ports C and D of the fiber optic loop coupler 5 travelclockwise and counter-clockwise along the fiber optic loop 6,respectively, and then return to the fiber optic loop coupler 5 throughports C and D and excite a coherent superposition therein; after thecoherent superposition, the linearly polarized light is again split intotwo beams by the fiber optic loop coupler 5 and are output from ports Aand B of the fiber optic loop coupler 5.

In the linearly polarized light output from port A of the fiber opticloop coupler 5: the linearly polarized light from port A travelingclockwise passes through each of the transmission arm and the couplingarm of the fiber optic loop coupler 5 once; and the linearly polarizedlight traveling counter-clockwise also passes through each of thetransmission arm and the coupling arm of the fiber optic loop coupler 5once. Therefore, the optical paths covered by the two beams of linearlypolarized light which travel clockwise and counter-clockwiserespectively from port A of the fiber optic loop coupler 5 are identicalwhen they return to port A of the fiber optic loop coupler 5, and thusthe linearly polarized light generated by their coherent superpositionare called reciprocal light, and the port outputting the reciprocallight is called reciprocal port. However, in the linearly polarizedlight output through port B of the fiber optic loop coupler 5: thelinearly polarized light traveling clockwise from port A of the fiberoptic loop coupler 5 passes through the transmission arm of the fiberoptic loop coupler 5 twice; and the linearly polarized light travelingcounter-clockwise passes through the coupling arm of the fiber opticloop coupler 5 twice. Therefore, the optical paths covered by the twobeams of linearly polarized light which travel clockwise andcounter-clockwise respectively from port A of the fiber optic loopcoupler 5 and reach port B of the fiber optic loop coupler 5 aredifferent, and thus the linearly polarized light generated by theircoherent superposition are called nonreciprocal light, and the portoutputting the nonreciprocal light is called nonreciprocal port.Nonreciprocal light signal cannot be used as detection signal of fiberoptic gyroscopes.

The linearly polarized light output through port A (the reciprocal port)of the fiber optic loop coupler 5 is input into port C of the lightsource coupler 3 through the polarizer 4, and the light source coupler 3splits the linearly polarized light signal input through port C into twobeams, wherein one beam is input into the photodetector 2 through port Bthereof. When the fiber optic loop 6 is stationary, the optical pathscovered by the two beams linearly polarized light which travel clockwiseand counter-clockwise respectively from port A of the fiber optic loopcoupler 5 are identical when they return to port A of the fiber opticloop coupler 5; and when the fiber optic loop 6 rotates, the opticalpaths covered by the two beams of linearly polarized light which travelclockwise and counter-clockwise respectively from port A of the fiberoptic loop coupler 5 are different when they return to port A of thefiber optic loop coupler 5; and under said two circumstances, theintensity of the optical signal received by the photodetector 2 differs,and thus the angular velocity of the rotation of the fiber optic loop 6can be calculated. This “minimum reciprocal configuration” has neverbeen challenged since the advent of the all-fiber interferometric fiberoptic gyroscope. References: (1) Hervé C. Lefèvre, “The Fiber-OpticGyroscope”, Artech House, Boston, 1993. (2) The Principles andTechnologies of Fiber-Optic Gyroscope, Zhang Guicai, National DefenseIndustry Press, 2008.

SUMMARY

This summary is provided to introduce in a simplified form certainconcepts that are further described in the Detailed Description belowand the drawings. This summary is not intended to identify essentialfeatures of the claimed subject matter or to limit the scope of theclaimed subject matter.

Introduced here is a new all-fiber interferometric fiber optic gyroscopehaving a minimum reciprocal configuration. A new all-fiberinterferometric fiber optic gyroscope having a minimum reciprocalconfiguration, as introduced here in, requires only five opticalelements. The light source coupler in FIG. 1 is a coupler in the generalsense and is exclusively used for splitting optical signal. According tothe characteristics of the fused taper PANDA polarization maintainingfiber optic coupler, the present invention includes a fused taper PANDApolarization maintaining fiber optic coupler which has the two functionsof simultaneously splitting the input optical signal and polarizing theoptical signal traveling along the transmission arm alone as the lightsource coupler such that the minimum reciprocal configuration of theall-fiber interferometric fiber optic gyroscope is reduced from sixoptical elements to five, with the polarizer (or depolarizer) removed.

The technical solution of the present invention according to oneembodiment is as follows:

An all-fiber interferometric fiber optic gyroscope having a minimumreciprocal configuration, characterized in that it comprises a polarizedlight source, a detection unit, a light source coupler, a fiber opticloop coupler, a polarization maintaining fiber optic loop; port A of thelight source coupler is counter-axially coupled to an output end of thepolarized light source, the other port of the light source coupler onthe same side as port A is coupled to the detection unit, port C on theother side of the light source coupler is counter-axially coupled to thefiber optic loop coupler, and the fiber optic loop coupler iscounter-axially coupled to the polarization maintaining fiber opticloop; wherein the light source coupler is for splitting input polarizedlight and polarizing the optical signal propagated along a transmissionarm alone, and ports A and C are two ports of the light source coupleron the same transmission arm.

Furthermore, the transmission arm of the light source coupler in oneembodiment has an output polarization extinction ratio greater than orequal to (≧) 20 dB.

Furthermore, the light source coupler can be a coupler manufactured by amethod for making a fused taper PANDA polarization maintaining fiberoptic coupler.

Furthermore, the light source coupler can be a 2×2 or 1×2 coupler.

Furthermore, the fiber optic loop coupler can be a 2×2 or 1×2 coupler.

Furthermore, the polarized light source can be a superluminescent diode.

Furthermore, the detection unit can be a photodetector diode.

Furthermore, the polarization maintaining fiber optic loop can be afiber optic coil wound with polarization maintaining optical fiber.

In comparison with the existing “minimum reciprocal configuration” ofall-fiber interferometric fiber optic gyroscope, the all-fiberinterferometric fiber optic gyroscope of the present invention is areduced minimum reciprocal configuration, and said minimum reciprocalconfiguration comprises: a superluminescent diode, a photodetectordiode, a light source coupler, a fiber optic loop coupler and a fiberoptic loop, altogether five optical elements, as shown in FIG. 2, andthe coupling manner thereof is provided as follows:

The superluminescent diode (which outputs elliptically or circularlypolarized light) is coupled to port A of the light source coupler, thephotodetector diode is coupled to port B of the light source coupler,port C of the light source coupler is coupled to port A of the fiberoptic loop coupler, port D of the light source coupler is void, ports Cand D of the fiber optic loop coupler are respectively coupled to thetwo ports of the fiber optic loop; and port B of the fiber optic loopcoupler is a nonreciprocal port and is void.

The light source coupler has the following characteristics: Thetransmission arm thereof has the two functions of splitting andpolarizing optical signal; and the coupling arm thereof merely has alight splitting function with respect to the optical signal. The lightsource coupler of the existing gyroscope configuration merely has alight splitting function, and the output light after light splitting ispolarized by the polarizer. The light source coupler in the presentinvention also differs from a polarizing beam splitter. Specifically, apolarizing beam splitter's transmission arm and the coupling arm thereofrespectively propagate beams of linearly polarized light whosepolarizing surfaces are perpendicular to each other, i.e., it canpolarize signals of circularly polarized light and ellipticallypolarized light, whereas it does not split linearly polarized light.

In comparison with the prior art, the advantageous effect of the presentinvention is as follows:

The present invention improves the minimum configuration of the fiberoptic gyroscope by reducing the traditional configuration comprising sixoptical elements to a configuration comprising five such that thestructure complexity of the all-fiber interferometric fiber opticgyroscope and the production cost are reduced, and the reliability ofthe gyroscope is enhanced, meanwhile, since the fuse splice is alsoreduced and the polarizer, removed, the optical loss is reduced suchthat the accuracy of the gyroscope is further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the optical path of a minimum reciprocalconfiguration of a typical all-fiber interferometric fiber opticgyroscope.

FIG. 2 is a schematic of the optical structure of an all-fiberinterferometric fiber optic gyroscope according to the presentinvention.

FIG. 3 shows the light source coupler of the present invention ingreater detail.

FIG. 4 illustrates polarization of light input to an optical fiber.

FIG. 5 shows a pair of curves illustrating light transmissioncharacteristics of the light source coupler according to an embodimentof the invention.

FIG. 6 illustrates a technique for making the light source coupleraccording to an embodiment of the invention.

DETAILED DESCRIPTION

As shown in FIGS. 2 and 3, the all-fiber interferometric fiber opticgyroscope of the present invention comprises: a superluminescent diode1, a photodetector diode 2, a light source coupler 12, a fiber opticloop coupler 5 and a polarization maintaining fiber optic loop 6,altogether five optical elements. Paths 13 and 14 between ports A-C andB-D, respectively, of the light source coupler 12 are transmission armswhich convert the input light into linearly polarized light and have alight splitting function to linearly polarized light. Paths 15 and 16between ports A-D and B-C, respectively, are coupling arms which merelyhave a light splitting function.

The superluminescent diode 1 can output circularly polarized light,elliptically polarized light or linearly polarized light and then inputthe polarized light output from the superluminescent diode 1 into anyone of ports A, B, C or D of the light source coupler 12, and thepresent embodiment selects port A as the input port, as shown in FIG. 2.The light source coupler 12 splits the input optical signal into twobeams and outputs them through ports A, B or C, D respectively. If theoptical signal is input through ports A, B, the split beams are outputfrom ports C, D; while if the optical signal is input through ports C,D, the split beams are output from ports A, B. The present embodimentselects ports C, D on the same side as the output ports, as shown inFIG. 2.

The optical signal output through the port in communication with theinput port is linearly polarized light (polarization extinction ratio≧20 dB), and the optical signal output through the port which is not incommunication with the input port is elliptically or circularlypolarized light. The elliptically or circularly polarized light is keptvoid, and the linearly polarized light can be input through any one ofA, B, C or D into the fiber optic loop coupler 5. The present embodimentselects port A of the fiber optic loop coupler 5 as the input port ofthe fiber optic loop coupler 5, as shown in FIG. 2. Port A of the lightsource coupler 12 coupled to the signal output port of thesuperluminescent diode 1 is explained below as an example, as shown inFIG. 2.

Port A of the light source coupler 12 is counter-axially coupled to thesignal output port of the superluminescent diode 1. The light sourcecoupler 12 has the function of splitting the input polarized light andpolarizing only the optical signal propagated along the transmissionarm. The light output through the output port C of the light sourcecoupler 12 is linearly polarized light. Port C of the light sourcecoupler 12 is counter-axially coupled to port A of the fiber optic loopcoupler 5, and the fiber optic loop coupler 5 splits the input linearlypolarized light signal into two beams of linearly polarized light havingthe same power and outputs them through ports C and D. The opticalsignals output through ports C and D of the fiber optic loop coupler 5are input through the two ports of the polarization maintaining fiberoptic loop 6 and travel clockwise and counter-clockwise, respectively.

The two beams of linearly polarized light traveling in oppositedirections along the polarization maintaining fiber optic loop 6 returnto the fiber optic loop coupler 5 through the fiber optic loop 6 andexcite a coherence superposition in the fiber optic loop coupler 5. Theoptical signal after the coherence superposition is then split into twobeams of linearly polarized light and are output through ports A and Bfrom the fiber optic loop coupler 5. In the linearly polarized lightoutput through port A from the fiber optic loop coupler 5: The linearlypolarized light signal emitted from port A of the fiber optic loopcoupler 5 which travels clockwise passes through each of thetransmission arm and the coupling arm of the fiber optic loop coupler 5once; and the linearly polarized light traveling counter-clockwise alsopasses through each of the transmission arm and the coupling of thefiber optic loop coupler 5 once. Therefore, the optical paths covered bythe beams of linearly polarized light traveling clockwise andcounter-clockwise from port A of the fiber optic loop coupler 5 areidentical when they return to port A of the fiber optic loop coupler 5without any difference, and thus are reciprocal. Thus, the linearlypolarized light generated by their coherent superposition is calledreciprocal light, and the port outputting the reciprocal light is calleda reciprocal port. However, in the linearly polarized light outputthrough port B of the fiber optic loop coupler 5: The linearly polarizedlight emitted from port A of the fiber optic loop coupler 5 andtraveling clockwise passes through the transmission arm of the fiberoptic loop coupler 5 twice; and the linearly polarized light travelingcounter-clockwise passes through the coupling arm of the fiber opticloop coupler 5 twice. Therefore, the optical paths covered by the beamsof linearly polarized light traveling clockwise and counter-clockwiserespectively from port A of the fiber optic loop coupler 5 are differentwhen they reach, port B of the fiber optic loop coupler 5, and thus thelinearly polarized light generated by their coherent superposition arecalled nonreciprocal light, and the port outputting the nonreciprocallight is called nonreciprocal port. Nonreciprocal light signal cannot beused as detection signal of fiber optic gyroscopes, and is thus keptvoid, while the reciprocal optical signal is input through port C of thelight source coupler 12.

The light source coupler 12 splits the optical signal input through portC into two beams, wherein one beam is input into the photodetector diodethrough port B of the light source coupler 12 and form a received signalof the all-fiber interferometric fiber optic gyroscope. The outputangular velocity of the all-fiber interferometric fiber optic gyroscopecan be obtained through demodulating, amplifying and processing thereceived signal.

It can be seen from FIG. 2 that the all-fiber interferometric fiberoptic gyroscope of the present invention comprises only five opticalelements.

The light source coupler 12 of the present invention is, in oneembodiment, a fused taper PANDA polarization maintaining fiber opticcoupler, characterized in that if the input optical signal of the lightsource coupler 12 is polarized light (linearly, circularly orelliptically polarized light), the optical signal output through thetransmission arm is linearly polarized light (polarization extinctionratio ≧20 dB), while the optical signal output through the coupling armis polarized light (linearly, circularly or elliptically polarizedlight, same as the input). The light source coupler 12 can bemanufactured by a method for making the existing fused taper PANDApolarization maintaining fiber optic coupler, or a suitable (i.e., acoupler simultaneously splitting the input optical signal and polarizingthe optical signal propagated through the transmission arm alone)coupler can be selected as the light source coupler in the presentinvention by detecting the existing light source couplers.

The fiber optic loop coupler 5 is a fused taper PANDA polarizationmaintaining fiber optic coupler, characterized in that if the inputoptical signal of the fiber optic loop coupler 5 is a linearly polarizedlight signal, the two beams of output optical signal are also linearlypolarized light.

The fiber optic loop 6 of the present invention is a fiber optic coilwound with PANDA polarization maintaining optical fiber, and it can alsoadopt fiber optic coil wound with other kinds of polarizationmaintaining optical fiber.

Referring now to FIG. 4, conventional fused-taper PMF couplers combineor split the input light and maintain their polarization states at theoutput ports. The splitting ratio of ports C and D is almost a constantwhen the input polarization angle θ is varied from 0° to 180° (i.e.,changing the input polarization states). The polarization sensitivity ofa conventional coupler should be as small as possible.

In contrast, FIG. 5 shows the characteristics of the light sourcecoupler 12 (e.g., a fused-taper PMF coupler) according to one embodimentof the present invention. The lower curve represents the output power atport C of light source coupler 12. The upper curve represents the outputpower at port D of light source coupler 12. The output powers of the PMFcoupler are dependent on the input polarization angle θ. This means thatthe coupling factor is not constant, but varies with input polarizationstates. For example, in one embodiment, when linearly polarized lightwith θ=0° is applied to port A of the light source coupler 12, as shownin FIG. 5, then ports C and D will output linearly polarized light witha splitting ratio PC(θ=0°)/PD(θ=0°) equal to about 44:56. Conversely,when linearly polarized light with θ=90° is applied to port A, almostall of the light energy passes through port D, while little output lightintensity is measured from port C. Thus, ports C and D will outputlinearly polarized light with a splitting ratio PC(θ=90°)/PD(θ=90°) ofabout 1:99 in that embodiment. Therefore, if we apply circular polarizedlight to port A, port C will output linearly polarized light withpolarization extinction ratio (PER) of about 10lg[PC(θ=0°)/PC(θ=90°)]=20 dB, and port D will output ellipticallypolarized light with PER of about 10 lg[PD(θ=0°)/PD(θ=90°)]=2 dB. Thismeans that for circular/elliptically polarized light, the transmissionarm (port A

port C and port B

port D) will act as an in-line PMF polarizer to translate the circularpolarized light into linearly polarized light with high PER. On theother hand, the coupling arm (port A

port D and port B

port C) will act as a conventional fused-taper PMF coupler for any kindsof polarized light.

A fused-taper PMF fiber coupler with these characteristics can beproduced as shown in FIG. 6. The light source coupler 12 can be producedby measuring the output light power of port C and port D while tuningthe radius R and the length L of the fused-taper section (e.g., byapplying a flame to heat the fused-taper section of the coupler 12)until 10 lg[PC(θ=0°)/PD(θ=90°)]>20 dB. In one embodiment L is in therange of 5 to 15 mm while R is in the range of 250 to 500 mm.

References in this specification to “an embodiment”, “one embodiment”,or the like, mean that the particular feature, structure orcharacteristic being described is included in at least one embodiment ofthe present invention. Occurrences of such phrases in this specificationdo not necessarily all refer to the same embodiment. On the other hand,different embodiments may not be mutually exclusive either.

Note that any and all of the embodiments described above can be combinedwith each other, except to the extent that it may be stated otherwiseabove or to the extent that any such embodiments might be mutuallyexclusive in function and/or structure.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be recognized that the inventionis not limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. Accordingly, the specification and drawings are to be regardedin an illustrative sense rather than a restrictive sense.

1. An all-fiber interferometric fiber optic gyroscope comprising: alight source; a light detector; a fiber optic loop coupler; apolarization maintaining fiber optic loop coupled to the fiber opticloop coupler; and a light source coupler coupled between the lightsource and the fiber optic loop coupler and coupled between the lightdetector and the fiber optic loop coupler, the fight source couplerhaving a plurality of ports and further having a plurality oftransmission arms and a plurality of coupling arms defined between pairsof the ports; wherein the light source coupler is constructed so as tosplit input polarized light, wherein the light source coupler is furtherconstructed so as to polarize an optical signal propagated along only atransmission arm and not to polarize an optical signal propagated alongany of the coupling arms.
 2. The gyroscope according to claim 1, whereinthe transmission arm has an output polarization extinction ratio greaterthan or equal to 20 dB.
 3. The gyroscope according to claim 2, whereinthe light source coupler is a coupler manufactured by a method formaking a fused taper PANDA polarization maintaining fiber optic coupler.4. The gyroscope according to claim 1, wherein the light source coupleris a coupler manufactured by a method for making a fused taper PANDApolarization maintaining fiber optic coupler.
 5. The gyroscope accordingto claim 4, wherein the light source coupler is a 2×2 or 1×2 coupler. 6.The gyroscope according to claim 1, wherein the fiber optic loop coupleris a 2×2 or 1×2 coupler.
 7. The gyroscope according to claim 1, whereinthe polarized light source is a superluminescent diode.
 8. The gyroscopeaccording to claim 1, wherein the detection unit is a photodetectordiode.
 9. The gyroscope according to claim 1, wherein the polarizationmaintaining fiber optic loop is a fiber optic coil wound withpolarization maintaining optical fiber
 10. An all-fiber interferometricfiber optic gyroscope comprising: a light source; a detection unit; afiber optic loop coupler; a polarization maintaining fiber optic loop,wherein the fiber optic loop coupler is counter-axially coupled to thepolarization maintaining fiber optic loop; and a light source couplerhaving a plurality of ports and having a plurality of transmission armsdefined between different pairs of the ports; wherein a first port ofthe light source coupler on a first side of the light source coupler iscounter-axially coupled to an output end of the polarized light source,a second port of the light source coupler on the first side of the lightsource coupler is coupled to the detection unit; wherein a third port ona second side of the light source coupler is counter-axially coupled tothe fiber optic loop coupler, wherein the light source coupler isconfigured to split input polarized light and to polarize only anoptical signal propagated along a transmission arm, and wherein thefirst and second ports are on the same transmission arm of the lightsource coupler.
 11. The gyroscope according to claim 10, wherein thetransmission arm of the light source coupler has an output polarizationextinction ratio greater than or equal to 20 dB.
 12. The gyroscopeaccording to claim 11, wherein the light source coupler is a couplermanufactured by a method for making a fused taper PANDA polarizationmaintaining fiber optic coupler.
 13. The gyroscope according to claim10, wherein the light source coupler is a coupler manufactured by amethod for making a fused taper PANDA polarization maintaining fiberoptic coupler.
 14. The gyroscope according to claim 13, wherein thelight source coupler is a 2×2 or 1×2 coupler.
 15. The gyroscopeaccording to claim 10, wherein the fiber optic loop coupler is a 2×2 or1×2 coupler.
 16. The gyroscope according to claim 10, wherein thepolarized light source is a superluminescent diode.
 17. The gyroscopeaccording to claim 10, wherein the detection unit is a photodetectordiode.
 18. The gyroscope according to claim 10, wherein the polarizationmaintaining fiber optic loop is a fiber optic coil wound withpolarization maintaining optical fiber.